1. Field of the Invention
The present invention relates to a laser irradiation method and particularly relates to a laser irradiation method being able to perform homogeneous annealing to an irradiated object. Moreover, the present invention relates to a method for manufacturing a crystalline semiconductor film and particularly relates to a method for manufacturing a crystalline semiconductor film having less variation of the crystallinity with high throughput.
2. Related Art
It is important to select a beam profile of a linear beam properly in order to give laser energy homogeneously to the irradiated object in a laser annealing process.
Since a beam spot of a laser beam emitted from a laser oscillator originally has wide energy distribution, the laser irradiation intensity is inhomogeneous according to the location in the irradiated object when the irradiated object is irradiated with the beam spot directly from the laser oscillator.
Consequently, a method has been employed in which an optical system including one or a plurality of cylindrical lens arrays is used to divide a beam spot and to superpose the divided beam spots so that the energy distribution of the beam spot is homogenized. Moreover, another method is suggested in which one or a plurality of cylindrical lenses is used in addition to the above optical system so as to form a linear beam having homogeneous energy distribution (refer to patent document 1).
When the laser annealing is performed using the beam spot having such homogeneous energy distribution, the irradiated object can be annealed homogeneously. [Patent document 1] Japanese Patent Laid-Open No. H10-153746.
However, even when the laser irradiation is performed under the same condition after adjusting the energy distribution of the beam spot as appropriate, the energy given to the irradiated object is not homogeneous yet. When a semiconductor film is crystallized to form a crystalline semiconductor film using such inhomogeneous irradiation energy, the crystallinity in the crystalline semiconductor film becomes inhomogeneous. As a result, when semiconductor elements are formed using this crystalline semiconductor film, their characteristics vary between the semiconductor elements.